Notes on Sex Determination, Environment, and Conservation
Recording and Privacy (Page 1)
Class recording purpose: educational use, revision, and for students unable to attend live sessions.
Recording scope: educator audio, video, main screen presentation including any video, guest presenters, and class activities; may also capture audio and video of class participants.
Access: recordings will be available through Moodle and should only be accessed by students enrolled in this unit.
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Privacy contact: Data Protection and Privacy Office, dataprotectionofficer@monash.edu.
Important note on Biological Sex vs Gender (Page 2)
This week’s workshop discusses genetic and physiological factors that contribute to biological sex, which is currently understood as a binary concept by the scientific community.
Gender is a separate social construct reflecting a person’s identity in society and will not be discussed in this session.
Animal Solutions to Life: Making more animals (Page 3)
Section title: "Animal Solutions to Life: Making more animals".
Presenter: Dr Kelly Merrin.
Acknowledgment of traditional lands (Page 4)
Acknowledgement: We are gathered on the traditional lands of the Kulin nations.
Respect: pay respects to Elders both past and present.
Learning Objectives (Page 5)
Compare and contrast the similarities and differences in modes of reproduction among different organisms.
Order the sequence of events required for sex determination in model organisms.
Propose how changes in the environment modify development by affecting hormones.
Discuss the impacts of environmentally-sensitive development on conservation and species management.
Sex determination in animals: It is complicated (Page 6)
Sex determination in animals is complex across taxa.
Key reference: Bachtrog et al., 2014.
Taxa mentioned: Beetles; Bees; Ants; Wasps; Butterflies; Moths; Flies.
Sex determination in animals: Modes (Page 7)
Three broad modes:
Sex chromosomes (cell-by-cell sex determination).
Sex chromosomes and hormones (mammals).
Environmentally-sensitive hormones (reptiles – active learning).
Diagram interpretation: Y chromosome vs. no Y chromosome (Page 8)
Core idea: Sex determination by sex chromosomes and hormones governs internal genitalia development.
With Y chromosome and testosterone:
Testosterone promotes development of Wolffian ducts; MĂĽllerian ducts degrade.
Leads toward male pathway; testes form.
Without Y chromosome and testosterone:
MĂĽllerian ducts develop into female internal genitalia (uterus, oviducts, vagina); Wolffian ducts regress.
Leads toward female pathway; ovaries form.
Anatomical structures mentioned: urinary bladder, urethra, testes, ovaries, uterus, vagina.
Summary: Y chromosome presence and hormone signals drive a male development pathway; absence drives a female pathway.
Vertebrate diversity and the evolution of sex determination (Pages 9–10)
Not all vertebrates do it the same way.
Phylogenetic notes (redrawn from Pask 2012):
160 million years ago (MYA): Evolution of Y-linked sex determination.
166 MYA: Linked sex determination emerged in some lineages.
315 MYA: Estrogen-directed ovarian development appears in some lineages.
Terms to track:
Genetically-directed ovarian development (in some vertebrates).
Estrogen-directed ovarian development (in non-mammalian vertebrates).
Not all vertebrates rely on Y-linked sex determination.
Visual cue: Not all vertebrates use the same mechanism; evolution has produced multiple pathways to determine sex.
Not all vertebrates do it the same way (Page 10)
Eutherian (placental) mammals:
Y chromosome involvement and hormones influence development; estrogen pathways can drive ovarian development.
Non-mammalian vertebrates:
Varied strategies; some rely less on a Y chromosome and may involve estrogen-directed ovarian development.
Concept to remember: Different lineages evolved different primary triggers for sex differentiation.
Sex determination in reptiles and amphibians: environmental influence (Page 11)
In reptiles and amphibians, sex determination is strongly influenced by the environment, particularly temperature.
Example species and temperature-genotype relationships (data shown in the figure):
Trachemys scripta: high temperatures bias toward males (often up to 100% males under particular temps).
Macroclemys temminckii: a different temperature response with a range of male proportions across temperatures (data points shown: 22–36 °C).
Alligator mississippiensis: sex ratios vary with temperature across a range (22–36 °C).
Note: The temperature axis on the figure is labeled in °C and shows specific transition points where sex ratio shifts occur.
Temperature effects on sex steroids (Page 12)
Question posed: What happens to sex steroids as temperature changes?
Key steroid pathway (simplified):
Cholesterol → Pregnenolone → DHEA → (Sex steroid precursors) → Progesterone → Testosterone → Estrone → Estradiol
Enzymes involved: aromatase; 17β-HSD (15 dihydroxy steroid dehydrogenase variants).
Temperature-sensitive elements:
Aromatase activity and other steroidogenic steps can be affected by temperature changes.
Important note from the slide: “This bit is unaffected” likely referring to a specific segment of steroidogenesis that remains stable across temperatures (the slide context implies partial independence for some components).
Aromatase activity and temperature (Page 14)
Aromatase activity is temperature-sensitive.
Data description (Benachour et al. 2007):
Aromatase activity measured as ext{pmol}\, ext{min}^{-1}\, ext{mg}^{-1} across a temperature range.
Temperature axis spans approximately 10 °C to 50 °C.
Activity values range from near 0 up to about 251 ext{pmol}\, ext{min}^{-1}\, ext{mg}^{-1}, showing a clear temperature-related pattern.
Implication: Temperature can modulate the balance of estrogen production via aromatase, influencing sex differentiation in temperature-sensitive species.
Chemicals in our environment and sex ratios (Pages 15–17)
Environmental exposure sources (illustrative, not exhaustive):
Hospital/medical facilities (HO), air pollutants (OH), agricultural pesticides, pharmaceutical industry waste, wastewater treatment plants, consumer products (e.g., toys).
Focus on endocrine-disrupting chemicals (EDCs) and sex ratios.
Specific groups studied:
Group 1: Fadrozole (a known aromatase inhibitor used in cancer treatment).
Group 2: Atrazine (a pesticide).
Related studies cited: Olmstead et al. 2009; Oka et al. 2008.
Proposed conceptual linkage:
Chemicals can influence the aromatase pathway and other steroidogenic steps, thereby shifting sex ratios in developing populations.
Mechanistic takeaway: Altering aromatase activity or steroid hormone signaling during development can bias offspring sex outcomes.
Mechanisms: how Fadrozole and Atrazine affect sex outcomes (Page 17)
Diagrammatic overview of steroid pathway components (simplified):
Cholesterol → Pregnenolone → DHEA → Progesterone → Testosterone → Estrone → Estradiol
Enzymes: Aromatase; 17β-HSD (17β-hydroxysteroid dehydrogenase).
Group 1: Fadrozole
Action: Aromatase inhibition → reduced estrogen synthesis.
Expected outcome: More males (
Empirical note on the slide: “Group 1: Fadrozole — More males”).
Group 2: Atrazine
Action: Influences aromatase pathway (and possibly other endocrine pathways) → increased estrogenic activity or disruption.
Expected outcome: More females (
Empirical note on the slide: “Group 2: Atrazine — More females”).
The note “This bit is unaffected” suggests that some parts of the pathway (likely upstream steroid precursors) remain unchanged under these exposures.
Your turn: group activity (Page 18)
Form groups of 4–6 people.
Each group is allocated one of two scenarios: Atrazine or Fadrozole.
Task: Prepare a hypothesis explaining why sex ratios might change in response to the assigned chemical exposure.
Delivery: Record your hypothesis in the appropriate Google Slide as a text box.
Discussion (Page 19)
Open discussion prompt: How do environmental chemicals influence development and sex ratios? What are the broader ecological and conservation implications?
Chemicals in our environment change aromatase activity (Page 20)
Studies cited: Yue and Brodie (1997) – effects on human choriocarcinoma cells.
Group 1: Fadrozole; Group 2: Atrazine.
Study reference: Fan et al. (2007) – human ovarian cells; findings related to aromatase inhibitors and estrogen synthesis.
Overall takeaway: Environmental chemicals can modulate aromatase activity and estrogen production, altering developmental trajectories linked to sex determination.
Implications in a changing world (Pages 21–24)
Temperature effects on sex ratios persist across species; graphs show shifts in the proportion of male hatchlings across nest temperature profiles.
Broader risk: Over 400 species are at risk due to temperature-dependent sex determination (TSD), including all sea turtles and crocodilians.
Visual examples: Chelonia mydas (Green Sea Turtle) illustration and related data.
Nested data themes:
Phase 1 vs. Phases 5 & 6 of the sex determination window.
Sex-determining period timing.
Nest temperature trends across months (Oct–Apr) and their relationship to hatchling outcomes.
Mortality and hatching success as functions of nest temperature and timing.
Implications in a changing world: conservation considerations (Page 23)
Image credit note: Chelonia mydas and related context.
Practical takeaway: Shifts in nest temperatures due to climate change can skew sex ratios, impacting population viability for species with TSD.
Conclusions (Page 25)
Key messages:
Genetics is not the sole determinant of sex in all animals.
Hormones and the environment play important roles in sex determination.
Anthropogenic (human-caused) activities influence sex ratios in certain species.
These changes have important implications for biodiversity conservation in the face of climate change.